A mixed group is a group on which one the layers comes from different sources,
then all the sub GeoMapFish layers (leaf) will be an OpenLayers layer.
By opposition a non mixed first level group contains only GeoMapFish layers WMS
from the same server, then we have only one OpenLayers layer for all the first level group.
All the group child will have the same value of his parent,
In other word, all the group of a first level group will have the same value.